1. Field of the Invention
The invention relates to a data transmission method, and more particularly to a hub device and a data transmission method.
2. Description of the Related Art
The Universal Serial Bus (USB), which is a standard for data transmission between a host device and a slave device, is commonly used in electronic devices. The data transmission bandwidth of USB 2.0 (also called high-speed USB) standard is 480 Mbps, and the data transmission bandwidth of USB 3.0 (also called Super Speed USB) is up to 5 Gbps.
Although USB 3.0 is an outstanding new standard, the USB 2.0 devices are still commonly used and continue to be prevalent on the market. Therefore, the USB 2.0 devices cannot be immediately replaced by the USB 3.0 devices. In this regard, the USB 3.0 host devices still have to support both USB 2.0 and 3.0 slave devices.
However, as defined in the USB 3.0 specifications, the USB 3.0 hub device includes a super speed sub-hub for supporting USB 3.0 data transmission and a USB 2.0 sub-hub for supporting USB 2.0 data transmission. Both of them are coupled between an upstream port for connecting to the host device and multiple downstream ports for connecting to the slave devices.
The upstream bus between the super speed sub-hub and the upstream port is USB 3.0 bus, and the upstream bus between the USB 2.0 sub-hub and the upstream port is USB 2.0 bus. When the USB 2.0 slave device is coupled to one of the downstream ports of the hub device, the data transmission path is formed by the USB 2.0 sub-hub through the USB 2.0 bus to the upstream port.
Therefore, when multiple USB 2.0 slave devices are concurrently coupled to the hub device, the slave devices share the 480 Mbps bandwidth of the upstream USB 2.0 bus. For example, when one USB 2.0 slave device is coupled to the hub device, the USB 2.0 slave device can use 480 Mbps bandwidth. When two USB 2.0 slave devices are coupled to the hub device, each USB 2.0 slave device may only use 480/2=240 Mbps bandwidth. When four USB 2.0 slave devices are coupled to the hub device, each USB 2.0 slave device may only use 480/4=120 Mbps bandwidth.
In other words, when multiple USB 2.0 slave devices are concurrently coupled to the hub device, the bandwidth that can be used is limited to the bandwidth of the upstream USB 2.0 bus, while the large bandwidth provided by the USB 3.0 bus between the super speed sub-hub and the upstream port is left unused.